1. Field of the Invention
The present invention relates to a wrist unit of an industrial robot which has three wrist elements.
2. Description of the Related Art
In a conventional known apparatus, an eccentric rocking-type reduction gear is provided at a joint of a wrist element of a front end side of a robot arm, and a torque of a motor is transmitted through the eccentric rocking-type reduction gear to the wrist element of the front end side. This type of apparatus is, for example, described in International Publication No. 2009/098945 (WO2009-098945A1). In the apparatus described in WO2009-098945A1, at the inside of the eccentric rocking-type reduction gear, a shaft for driving a joint at a front end side from the eccentric rocking-type reduction gear (hereinafter referred to as a “through shaft” for convenience) is passed through. Drive power from the motor is input to the through shaft via a gear. The eccentric rocking-type reduction gear has crankshafts at positions offset from the center axial line in the radial direction. One of the crankshafts has an input gear at its end. The input gear of the crankshaft engages with a motor gear which is arranged at a position offset from the crankshaft in the radial direction. Drive power from the motor is input through the motor gear and the input gear to the crankshaft.
In the apparatus described in WO2009-098945 A1, the center axial line of the opposite side gear (motor gear) which engages with the input gear of the reduction gear at the end of the crankshaft cannot be arranged on the same axial line as the center axial line of the reduction gear. It is for this reason that only one of the plurality of crankshafts is provided at its end with an input gear for use as an input shaft. In this configuration, the load concentrates at the crankshaft which has the input gear, so the crankshaft and the bearings which support it have to be configured so as to be able to withstand that load. For this reason, the crankshaft and the bearings which support it become larger in size and the reduction gear as a whole becomes larger in size. Furthermore, since the opposite side gear which engages with the input gear of the reduction gear cannot be arranged with its center axial line coaxial with the center axial line of the reduction gear, when positioning the base end side of the reduction gear, it is necessary to position the center axial line of the reduction gear and then match the phase of the reduction gear so that the input gear of the reduction gear and opposite side gear engage. For this reason, separate parts used for matching the phase become necessary, and assembly and disassembly of the reduction gear become complicated.